This work was supported in part by the National Natural Science Foundation of China (60825303, 60834003), in part by the 973 Project (2009CB320600), in part by the Research Fund for the Doctoral Programme of Higher Education of China (20070213084), in part by the Heilongjiang Outstanding Youth Science Fund (JC200809), in part by Postdoctoral Science Foundation of China (200801282), in part by the Fok Ying Tung Education Foundation (111064), and in part by the Research Grants Council under HKU 7031/07P.